This project will establish biogeochemical indicators for methylmercury production that will be used to improve the effectiveness of in-situ remediation of mercury-contaminated sediments. Methylmercury (MeHg) is a potent neurotoxin that is produced by anaerobic microorganisms and biomagnifies in the aquatic food web. In-situ remediation methods such as sediment amendments have not been widely implemented, mainly because the effects on MeHg production are unknown. Moreover, the processes that control mercury methylation can differ between sites, and this uncertainty is a barrier for effective implementation of in-situ remediation. This project aims to address this knowledge gap by establishing indicators of mercury methylation potential and using them to assess the effectiveness of sediment amendments for remediation. The research will focus on two critical drivers of methylmercury production: the environmental conditions that promote the growth of sediment microorganisms that produce MeHg and the processes that influence the bioavailability of mercury for these microorganisms. The balance between these two drivers will determine the success of in-situ sediment treatments. In this respect, the central hypothesis of this work is that the identification of the primary controls to methylmercury production at the fied site will improve the efficacy of sediment treatments. To test this hypothesis, the research team will apply fundamental knowledge regarding mercury biogeochemistry to improve the determination of mercury methylation potential in sediments and compare in-situ techniques for sediment remediation. Specific aims are to: (1) Develop relationships between the activity of methylating microorganisms and MeHg production in sediments; (2) Refine methods to quantify mercury bioavailability in sediments; (3) Identify factors that control methylation potential and formulate guidelines for characterization of sites; and (4) Determine how sediment amendment techniques influence methylmercury production potential. This work will be implemented with laboratory sediment microcosm experiments that will simulate a range of conditions relevant to mercury-contaminated Superfund sites. A variety of molecular biological and geochemical tools will be used to quantify the activity of methylating microorganisms and the reactivity of mercury as related to biouptake into anaerobic microorganism. The novelty of this work is that the researchers will apply recent discoveries to develop new tools to quantify mercury methylation potential. These discoveries include the genetic basis of mercury methylation in microorganisms and the geochemical processes controlling mercury speciation in sediments. The researchers will work with managers of Superfund sites in the implementation of the research, interpretation of the results, and also in the establishment of a guiding framework for assessments at specific field sites and the selection of remediation strategies.